Hitherto the lapping ability of water-based decorative paints has been inferior to that of organic solvent-based paints, especially solvent-based paints comprising alkyd resin and white spirit. This inferiority is particularly noticeable with gloss or partial gloss paints, that is to say paints which dry to produce a coating having a specular gloss at an angle of 60.degree. of at least 30% (or at least 65% for high gloss) where specular gloss at 60.degree. is measured according to ASTM Test D523 using light reflected at an angle of 60.degree. to the normal to the surface of the paint. Water-based air-drying gloss paints have been proposed which comprise water and an aqueous colloidal dispersion of water-insoluble film-forming polymer (including copolymers) such as for example a polymer or co-polymer of alkyl acrylate (including methacrylate). When such paints dry on a surface, the particles of polymer begin to coalesce to form a coherent film of coalesced particles which coats the surface but which dries so quickly that the particles of film-forming polymer begin to coalesce too quickly. This means that in practice there is not always time for a band of paint to lap with a subsequently applied adjacent contiguous band. Failure to lap creates unsightly evidence of boundaries in a coating and especially in a coating of the type which dries to produce a gloss or partial gloss finish.
Attempts were made to improve lapping ability by slowing down the speed at which the water-based paint dried. This was done by adding high boiling solvent to the water. Some improvement was achieved, but the lapping ability was still unacceptably low, falling well short of what could be achieved from organic solvent-based paint. In addition the slowing down of drying is in itself undesirable because wet coatings ought to dry as quickly as possible. Usually a wet coating should become dry to the touch after 10 (preferably after 6) hours at ambient temperatures and dry enough to receive a further coating usually after 48 (preferably after 16) hours from application.
British Patent Specifications GB 1 476 584 and GB 2 053 934A disclose water-based autoxidisable air-drying coating compositions which contain as their binder material a neutralised copolymer of for example acrylic acid with alkyl acrylate and/or methacrylate and also a comonomer which is a condensate of glycidyl or hydroxyalkyl ester of acrylic acid with a long chain unsaturated fatty acid such as linseed oil fatty acid. The unsaturation in the acrylic ester portion of the condensate allows the condensate to copolymerise with the acrylic acid and alkyl acrylate or methacrylate comonomers whilst the portion of the condensate derived from the long chain fatty acid confers autoxidisability to the copolymeric binder material. Unfortunately, this portion derived from the long chain fatty acid is highly hydrophobic and so a high concentration of neutralising ions is needed to make the copolymer soluble in water and as a result the dried paints are too easily disfigured by water.
Preferably coating compositions should also have a good resistance to discolouration (more particularly yellowing) with age.
An object of this invention is to provide a water-based autoxidisable air-drying coating composition suitable for application at ambient temperatures which laps well yet dries quickly in air and achieves an adequate resistance to disfiguration by water. A refinement of the invention provides a coating composition having improved resistance to discolouration on ageing. Another refinement of the invention provides a coating composition having improved gloss. Another objective of the invention is to provide a partially esterified carboxylic acid film-forming copolymer which is suitable for use in coating compositions of this invention when suitably neutralised.
A further refinement of the invention is provided by the discovery that dynamic solution viscosity as measured in 1-methoxy-2-hydroxy propane (MHP) by an oscillatory parallel plate rheometer can be used to select certain specified partially esterified carboxylic acid film-forming copolymers which enable the above objectives to be met. MHP is ##STR1##
These and other advantages of this invention will become more apparent by reference to the detailed description and the illustrative examples.